Gelled aqueous composition comprising a block copolymer containing at least one water-soluble block and one hydrophobic block

ABSTRACT

The invention relates to a gelling aqueous composition comprising block copolymers containing at least one water-soluble block and at least one block predominantly hydrophobic in nature, and forming a viscoelastic gel.

[0001] The present invention relates to a gelled aqueous compositioncomprising a block copolymer containing at least one water-soluble blockand one hydrophobic block.

[0002] Amphiphilic molecules are molecules having differentwater-solubility regions which give them special properties. A knownexample of an amphiphilic molecule is that of surfactants which may havea hydrophilic and a hydrophobic region.

[0003] Because of their amphiphilic character, these molecules cometogether and organize themselves in solution in water to form micelles.These micelles may be of various morphologies, such as sphericalmicelles or anisotropic micelles (for example lamellar or vermicularmicelles). Spherical micelles are the most common as they are the mostaccessible.

[0004] These micelles are in equilibrium, which means that dilution oraddition of a solvent or of a cosurfactant to the medium containingthese micelles results in a variation in the size of the micelles or intheir morphology.

[0005] One objective of the present invention is to provide amphiphilicblock copolymers of hydrophobic/hydrophilic structure which can form agel when they are in water.

[0006] Another objective is to be able to obtain easily preparableaqueous gels whose elastic modulus can be adjusted.

[0007] To achieve the above objectives, the purpose of the invention isthe use of a block copolymer containing at least one block water-solublein nature and at least one block predominantly hydrophobic in nature,which copolymer is in the form of micelles when it is in water.

[0008] This block copolymer forms a viscoelastic gel when it is insolution in water.

[0009] This block copolymer contains at least one block predominantlyhydrophobic in nature and at least one water-soluble block, thepredominantly hydrophobic block having hydrophilic units preferably inan amount of less than 33% by weight with respect of the total weight ofthe units of said predominantly hydrophobic block. This amount may beequal to 0 but is preferably at least 1% by weight and less than 25% byweight, even more preferably between 2 and 15%, with respect to thetotal weight of the units of said predominantly hydrophobic block.

[0010] This block copolymer contains at least one block predominantlyhydrophobic in nature and at least one water-soluble block, thewater-soluble block having hydrophobic units in an amount which may besmall, about 1% of the total weight of the units of said water-solubleblock. The maximum amount of hydrophobic units depends on the nature ofthe units and is in most cases less than 70% by weight and at least 1%by weight, and even more preferably less than 50% by weight and at least10%, with respect to the total weight of the units of said water-solubleblock.

[0011] The invention also relates to a process for preparing these blockcopolymers by so-called living or controlled polymerization.

[0012] The invention also relates to a process for controlling thehydrophilic/hydrophobic balance of amphiphilic block copolymers havingat least one block coming from the polymerization of hydrophilicmonomers and at least one block coming from the polymerization ofhydrophobic monomers, in which process:

[0013] hydrophilic units are introduced into the block coming from thepolymerization of hydrophobic monomers, and/or

[0014] hydrophobic units are introduced into the block coming from thepolymerization of hydrophilic monomers.

[0015] Finally, the invention relates to the use of these blockcopolymers as gelling agents or as thickeners for aqueous medium.

[0016] The invention firstly therefore relates to a block copolymercontaining at least one block water-soluble in nature and at least oneblock predominantly hydrophobic in nature. According to a firstembodiment, the copolymer contains only a single hydrophobic block and asingle water-soluble block. According to another embodiment, thecopolymer contains a water-soluble block having a hydrophobic group ateach end or vice-versa.

[0017] In the description which follows, the expression “blockwater-soluble in nature” should be understood to mean a polymer blockcontaining a number of hydrophilic groups sufficient to obtain a watersoluble block well dissolved in water. Solubility in water of the watersoluble block means a block copolymer containing such a water-solubleblock, when mixed with water, gives a translucent monophasic system.Usually such a translucent monophasic system is obtained from a watersoluble block comprising at least 30%, preferably at least 50% by weightof hydrophilic units with respect to the totality of units of thewater-soluble block. The block water-soluble in nature is thereforesoluble in water. The term “unit” should be understood to mean that partof the block corresponding to a monomeric unit.

[0018] Likewise, the expression “block predominantly hydrophobic innature” should be understood to mean a polymer block preferablycontaining at least 67% by weight hydrophobic units with respect to thetotality of units. The block predominantly hydrophobic in nature is notsoluble in water. This block copolymer containing at least one blockwater-soluble in nature and at least one block predominantly hydrophobicin nature forms a viscoelastic gel when it is in solution in water.

[0019] The term “viscoelastic gel” should be understood to mean a liquidmedium for which the viscous modulus G″ and the elastic modulus G′ aresuch that G′>G″. This gel behaviour is manifested by a flow thresholdand even, in some cases, by a shear-thickening effect (an increase inthe viscosity with flow). This gel effect is obtained when the polymerconcentration exceeds a certain threshold called the critical gellingconcentration.

[0020] The block copolymers according to the present invention have theadvantage of making the aqueous media viscoelastic when they are used inonly a small amount with respect to the aqueous medium. The copolymer ispreferably used at a concentration higher than 0.1% by weight and evenmore preferably at a concentration from 1 to 10% by weight.

[0021] The properties of the copolymers according to the presentinvention may be obtained by selecting the nature of the soluble blocksand the nature of the predominantly hydrophobic blocks, at least thehydrophilic block having to contain hydrophobic groups in an appropriateamount.

[0022] According to one embodiment of the invention, the weight ratio ofthe block water-soluble in nature to the completely hydrophobic block isbetween 95/5 and 20/80, even more preferably between 90/10 and 40/60.

[0023] According to a first version of the preparation, the blockswater-soluble in nature and the blocks predominantly hydrophobic innature of the above copolymers may come from the copolymerization ofhydrophilic and hydrophobic monomers. The amounts of hydrophilic andhydrophobic units in each of the said blocks can then be controlled bythe respective contents of hydrophilic monomers and hydrophobic monomersduring the polymerization of the blocks.

[0024] Thus, the blocks predominantly hydrophobic in nature may comefrom the copolymerization of hydrophobic monomers and of hydrophilicmonomers, the hydrophilic monomers being present in an amount of lessthan 33% by weight, preferably at least 1% by weight, even morepreferably between 2 and 15%, with respect to the total weight of theunits of the hydrophobic block.

[0025] In addition, the blocks water-soluble in nature may come from thecopolymerization of hydrophilic monomers and of hydrophobic monomers,the hydrophobic monomers being present in an amount of less than 70% byweight, preferably at least 1% by weight, even more preferably between50 and 25%, with respect to the total weight of the units of thewater-soluble block.

[0026] According to a second version of the preparation, the blockswater-soluble in nature may come:

[0027] from the polymerization of monomers that may be renderedhydrophilic by hydrolysis and optionally of non-hydrolysable hydrophobicmonomers and/or of hydrophilic monomers, and then

[0028] from the hydrolysis of the polymer obtained.

[0029] During the hydrolysis, the units corresponding to thehydrolysable monomers are hydrolysed into hydrophilic units.

[0030] The amounts of hydrophilic and hydrophobic units in each of thesaid blocks are then controlled by the amount of each type of monomerand by the degree of hydrolysis.

[0031] According to this second version, various methods ofimplementation may be envisaged.

[0032] According to a first method of implementation, the blocks may beobtained by:

[0033] homopolymerization of hydrophobic monomers that can be renderedhydrophilic by hydrolysis and

[0034] partial hydrolysis of the homopolymer obtained to a degree suchthat what is obtained is:

[0035] either, in the case of the blocks predominantly hydrophobic innature, an amount of hydrophilic units of less than 33% by weight,preferably at least 1% by weight, even more preferably between 2 and15%, with respect to the total weight of the units of the hydrophobicblock.

[0036] or, in the case of the blocks water-soluble in nature, an amountof hydrophobic units of less than 70% by weight, preferably at least 1%by weight, even more preferably between 25 and 50%, with respect to thetotal weight of the units of the water-soluble block.

[0037] According to a second method of implementation, the blocks may beobtained by:

[0038] copolymerization of hydrophobic monomers that can be renderedhydrophilic by hydrolysis and of hydrophobic monomers that cannot berendered hydrophilic by hydrolysis and then

[0039] complete or partial hydrolysis of the polymer obtained.

[0040] According to this second method of implementation, the amount ofhydrophilic and hydrophobic units may depend on two criteria, namely thecontent of the various types of monomers and the degree of hydrolysis.

[0041] If there is complete hydrolysis, it is sufficient to vary thecontent of the monomers and thus:

[0042] the blocks predominantly hydrophobic in nature can come:

[0043] from the polymerization of a mixture of hydrophobic monomers thatcan be rendered hydrophilic by hydrolysis and of hydrophobic monomersthat cannot be rendered hydrophilic by hydrolysis, the hydrophobicmonomers that can be rendered hydrophilic by hydrolysis being present inan amount of less than 33% by weight, preferably at least 1% by weight,even more preferably between 2 and 15%, with respect to the total weightof the units of the hydrophobic block, and then

[0044] from the complete hydrolysis of the polymer obtained;

[0045] the blocks water-soluble in nature may come:

[0046] from the polymerization of a mixture of hydrophobic monomers thatcan be rendered hydrophilic by hydrolysis and of hydrophobic monomersthat cannot be rendered hydrophilic by hydrolysis, the hydrophobicmonomers that cannot be rendered hydrophilic by hydrolysis being presentin an amount of less than 70% by weight, preferably at least 1% byweight, even more preferably between 50 and 25%, with respect to thetotal weight of the units of the water-soluble block, and then

[0047] from the complete hydrolysis of the polymer obtained.

[0048] If there is partial hydrolysis, the monomer content and thedegree of hydrolysis may be varied at the same time.

[0049] According to a third method of implementation, the blocks may beobtained by:

[0050] copolymerization of hydrophobic monomers that can be renderedhydrophilic by hydrolysis and of hydrophilic monomers and then

[0051] partial hydrolysis of the polymer obtained to a degree such thatwhat is obtained is:

[0052] either, in the case of the blocks predominantly hydrophobic innature, an amount of hydrophilic units of less than 33% by weight,preferably at least 1% by weight, even more preferably between 2 and15%, with respect to the total weight of the units of the hydrophobicblock.

[0053] or, in the case of the blocks water-soluble in nature, an amountof hydrophobic units of less than 70% by weight, preferably at least 1%by weight, even more preferably between 50 and 25%, with respect to thetotal weight of the units of the water-soluble block.

[0054] In general, the hydrophobic monomers may be chosen from:

[0055] vinylaromatic monomers, such as styrene,

[0056] dienes, such as butadiene,

[0057] alkyl acrylates and methacrylates the alkyl group of whichcontains from 1 to 10 carbon atoms, such as methyl, ethyl, n-butyl,2-ethylhexyl, tert-butyl, isobornyl, phenyl and benzyl acrylates andmethacrylates.

[0058] Preferably, it is styrene.

[0059] The hydrophilic monomers may be chosen from:

[0060] ethylenically unsaturated carboxylic acids such as acrylic andmethacrylic acids;

[0061] neutral hydrophilic monomers such as acrylamide and itsderivatives (N-methylacrylamide, N-isopropylacrylamide), methacrylamide,polyethylene glycol methacrylate and polyethylene glycol acrylate;

[0062] anionic hydrophilic monomers: sodium2-acrylamido-2-methylpropanesulphonate (SAMPS), sodium styrenesulphonateand sodium vinylsulphonate.

[0063] The monomers that can be rendered hydrophilic by hydrolysis maybe chosen from:

[0064] acrylic and methacrylic esters hydrolysable in acid, such asmethyl acrylate, ethyl acrylate, hydroxyethyl methacrylate, hydroxyethylacrylate and tert-butyl acrylate;

[0065] vinyl acetate hydrolysable into vinyl alcohol units;

[0066] quaternized 2-dimethylaminoethyl methacrylate and acrylate(quatdamma and quatdama);

[0067] acrylamide and (meth)acrylamide.

[0068] Preferably, the block copolymers according to the invention arediblock copolymers. However, they may also be triblock, or evenmultiblock copolymers. If the copolymer comprises three blocks, it ispreferable to have a block water-soluble in nature flanked by two blockspredominantly hydrophobic in nature.

[0069] According to a particular embodiment of the invention, thecopolymer is a diblock copolymer comprising a block water-soluble innature and a block predominantly hydrophobic in nature, in which:

[0070] the block water-soluble in nature contains acrylic acid (AA)units and ethyl acrylate (EtA) units and

[0071] the block predominantly hydrophobic in nature contains styrene(St) units and methacrylic acid (MAA) and/or hydroxyethyl methacrylate(HEMA) units.

[0072] Preferably, according to this embodiment, the block water-solublein nature comes:

[0073] from the polymerization of methacrylic acid (MAA) and of ethylacrylate (EtA) in an EtA/MAA weight ratio from 90/10 to 99/1, and then

[0074] from the hydrolysis of the polymer obtained to a degree of atleast 50 mol % up to 95 mol %.

[0075] Preferably, the block predominantly hydrophobic in nature comesfrom the polymerization of a monomer mixture comprising at least 80% byweight styrene.

[0076] Generally, the block copolymers according to the invention have amolecular mass of at most 100,000 g/mol, preferably at least 1000 g/mol.

[0077] In general, the above block copolymers can be obtained by anyso-called living or controlled polymerization process such as, forexample:

[0078] radical polymerization controlled by xanthates according to theteaching of Application WO 98/58974,

[0079] radical polymerization controlled by dithioesters according tothe teaching of Application WO 97/01478,

[0080] polymerization using nitroxide precursors according to theteaching of Application WO 99/03894,

[0081] radical polymerization controlled by dithiocarbamates accordingto the teaching of Application WO 99/31144,

[0082] atom transfer radical polymerization (ATRP) according to theteaching of Application WO 96/30421,

[0083] radical polymerization controlled by iniferters according to theteaching of Otu et al., Makromol. Chem. Rapid. Commun., 3, 127 (1982),

[0084] radical polymerization controlled by degenerative iodine transferaccording to the teaching of Tatemoto et al., Jap. 50, 127, 991 (1975),Daikin Kogyo Co Ltd., Japan and Matyjaszewski et al., Macromolecules,28, 2093 (1995),

[0085] group transfer polymerization according to the teaching of O. W.Webster “Group Transfer Polymerization”, pp. 580-588 in “Encyclopedia ofPolymer Science and Engineering”, vol. 7 and H. F. Mark, N. M. Bikales,C. G. Overberger and G. Menges, Publ., Wiley Interscience, New York,1987,

[0086] radical polymerization controlled by tetraphenylethanederivatives (D. Braun et al., Macromol.Symp. 111,63 (1996)), and

[0087] radical polymerization controlled by organocobalt complexes(Wayland et al., J.Am.Chem.Soc. 116,7973 (1994)).

[0088] The preferred polymerization is living radical polymerizationusing xanthates.

[0089] The invention therefore furthermore relates to a process forpreparing these block copolymers. This process consists in:

[0090] 1• the following being brought into contact with one another:

[0091] at least one ethylenically unsaturated monomer,

[0092] at least one source of free radicals and

[0093] at least one compound of formula (I):

[0094]  in which:

[0095] R represents an R²O—, R²R′²N— or R³— group, where: R² and R′²,which are identical or different, represent (i) an alkyl, acyl, aryl,alkene or alkyne group or (ii) a saturated or unsaturated, possiblyaromatic, carbocycle or (iii) a saturated or unsaturated heterocycle,these groups and rings (i), (ii) and (iii) possibly being substituted,

[0096] R³ represents H, Cl, an alkyl, aryl, alkene or alkyne group, asaturated or unsaturated, optionally substituted (hetero) cycle, analkylthio, alkoxycarbonyl, aryloxycarbonyl, carboxy, acyloxy, carbamoyl,cyano, dialkylphosphonato, diarylphosphonato, dialkylphosphinato ordiarylphosphinato group, or a polymer chain,

[0097] R¹ represents (i) an optionally substituted alkyl, acyl, aryl,alkene or alkyne group or (ii) an optionally substituted or aromatic,saturated or unsaturated, carbocycle or (iii) an optionally substituted,saturated or unsaturated, heterocycle, or a polymer chain;

[0098] 2• the above contacting operation being repeated at least once,using:

[0099] monomers differing from those in the previous operation, and

[0100] instead of the precursor compound of formula (I), the polymercoming from the previous operation; and

[0101] 3• optionally, the copolymer obtained being hydrolysed.

[0102] The R¹, R², R^(′2) and R³ groups may be substituted with alkylgroups, substituted phenyls, substituted aromatic groups or one of thefollowing groups: oxo, alkoxycarbonyl or aryloxycarbonyl (—COOR),carboxy (—COOH), acyloxy (—O₂CR), carbamoyl (—CONR₂), cyano (—CN),alkylcarbonyl, alkylarylcarbonyl, arylcarbonyl, arylalkylcarbonyl,isocyanate, phthalimido, maleimido, succinimido, amidino, guanidimo,hydroxyl (—OH), amino (—NR₂), halogen, allyl, epoxy, alkoxy (—OR),S-alkyl, S-aryl, silyl, groups having a hydrophilic or ionic character,such as alkali metal salts of carboxylic acids, alkali metal salts ofsulphonic acid, polyoxy alkylene (POE, POP) chains, and cationicsubstituents (quaternary ammonium salts), R representing an alkyl oraryl group.

[0103] Preferably, the compound of formula (I) is a dithiocarbonatechosen from compounds of the following formulae (IA), (IB) and (IC):

[0104] in which:

[0105] R² and R²′ represent (i) an alkyl, acyl, aryl, alkene or alkynegroup, or (ii) an optionally aromatic, saturated or unsaturated,carbocycle or (iii) a saturated or unsaturated heterocycle, these groupsand rings (i), (ii) and (iii) possibly being substituted;

[0106] R¹ and R^(1′) represent (i) an optionally substituted alkyl,acyl, aryl, alkene or alkyne group or (ii) an optionally substituted oraromatic, saturated or unsaturated, carbocycle or (iii) an optionallysubstituted, saturated or unsaturated, heterocycle, or a polymer chain;

[0107] p is between 2 and 10.

[0108] During step 1, a first block of the copolymer is synthesized soas to become water soluble or hydrophobic in nature depending on thenature and the amount of monomers used. During step 2, the other blockof the polymer is synthesized.

[0109] The ethylenically unsaturated monomers will be chosen from thehydrophilic, hydrophobic and hydrolysable monomers defined above, inproportions suitable for obtaining a block copolymer whose blocks havethe characteristics of the invention. According to this process, if allthe successive polymerization steps are carried out in the same reactor,it is generally preferable for all the monomers used during one step tohave been consumed before the polymerization of the next step starts,therefore before the new monomers have been introduced. However, it mayhappen that the hydrophobic or hydrophilic monomers of the previous stepare still present in the reactor during the polymerization of the nextblock. In this case, these monomers generally represent no more than 5mol % of all the monomers and they participate in the followingpolymerization by contributing to introducing hydrophobic or hydrophilicunits into the next block.

[0110] For more details with regard to the above polymerizationprocesses, the reader may refer to the contents of Application WO98/58974.

[0111] The hydrolysis may be carried out using a base or an acid. Thebase may be chosen from alkali or alkaline-earth metal hydroxides, suchas sodium hydroxide or potassium hydroxide, alkali metal alcoholates,such as sodium methylate, sodium ethylate, potassium methylate,potassium ethylate and potassium tert-butylate, ammonia and amines suchas triethylamines. The acids may be chosen from sulphuric acid,hydrochloric acid and paratoluenesulphonic acid. It is also possible touse an ion-exchange resin or an ion-exchange membrane of the cationic oranionic type. The hydrolysis is generally carried out a temperature ofbetween 5 and 100° C., preferably between 15 and 90° C.

[0112] After hydrolysis, the block copolymer can be washed, for exampleby dialysis against water, or using a solvent such as alcohol. It mayalso be precipitated by lowering the pH below 4.5.

[0113] The hydrolysis may be carried out on a monoblock polymer, whichwill then be linked to other blocks, or on the final block copolymer.

[0114] The invention also relates to a process for controlling thehydrophilic/hydrophobic balance of amphiphilic block copolymers havingat least one block coming from the polymerization of hydrophilicmonomers and at least one block coming from the polymerization ofhydrophobic monomers, in which:

[0115] hydrophilic units are introduced into the block coming from thepolymerization of hydrophobic monomers, and/or

[0116] hydrophobic units are introduced into the block coming from thepolymerization of hydrophilic monomers.

[0117] Finally, the invention relates to the use of the above blockcopolymers as a gelling agent or as a thickening agent in aqueous andorganic media. Preferably, the polymers have to be used in aconcentration of at least 0.1% by weight and of at most 20%, even morepreferably 0.5 to 10% by weight in said aqueous and organic media. Theblock copolymers according to the invention therefore have the advantageof allowing gelling in liquid media by being used in very lowconcentration. Consequently, the cost of using them is lower and theyhave little or no influence on the properties of the gelled medium.

[0118] The following examples illustrate the invention without howeverlimiting its scope.

EXAMPLES

[0119] In the examples which follow:

[0120] M_(n), represents the number-average molecular mass of thepolymers, M_(n), being expressed in polystyrene equivalents (g/mol),

[0121] M_(w) represents the weight-average molecular mass,

[0122] M_(w)/M_(n) represents the polydispersity index,

[0123] the polymers, before hydrolysis, are analysed in GPC with THF asthe elution solvent.

A—Synthesis of the Block Copolymers (Examples 1 to 7)

[0124] For all the following examples, the polymerizations are carriedout to a degree of conversion of the monomers of greater than 95%.

Example 1

[0125] Synthesis and Hydrolysis of a poly(styrene/methacrylicacid/2-hydroxyethyl methacrylate)-b-poly(ethylacrylate/methacrylic acid)Diblock Copolymer

[0126] 1.1. Synthesis of a Random styrene/methacrylicacid/2-hydroxyethyl methacrylate Copolymer. Mass Ratios:St/MAA/HEMA=90/5/5.

[0127] The polymerization was carried out in emulsion, in a jacketedreactor fitted with a stainless steel three-bladed stirrer. Introducedas a stock into the reactor, at room temperature, were 1178 g of waterand 25.36 g of dodecyl sulphate (Texapon K12/96). The mixture obtainedwas stirred for 30 minutes (at 175 rpm) under nitrogen. The temperaturewas then raised to 85° C. and then 1.55 g of ammonium persulphate(NH₄)₂S₂O₈ in 2.48 g of water were incorporated.

[0128] At the same time, a mixture comprising:

[0129] 248 g of styrene (St),

[0130] 13.95 g of methacrylic acid (MAA),

[0131] 13.95 g of 2-hydroxyethyl methacrylate (HEMA), and

[0132] 7.44 g of methyl α-(O-ethylxanthyl)-propionate (CH₃CHCO₂Me)SCSOEt(composed of formula IA) started to be added.

[0133] The addition lasted 55 minutes. 15 minutes after starting to addthe mixture comprising the monomers and the methylα-(O-ethylxanthyl)propionate, 0.56 g of sodium carbonate Na₂CO₃dissolved in 100 g of water started to be added. The latter additiontook place over 45 minutes.

[0134] After the various ingredients had been completely added, anemulsion polymer (latex) was obtained which was maintained at 85° C. forone hour. After cooling to room temperature, 91 g of the polymeremulsion were removed for analysis.

[0135] The analysis results were as follows:

[0136] M_(n)=5900 g/mol

[0137] M_(w)/M_(n)=2.2

[0138] 1.2. Synthesis of the Diblock Copolymer

[0139] The synthesis started with the emulsion copolymer obtained above(section 1.1.). To this were added at 85° C., over one hour:

[0140] 308 g of ethyl acrylate (EtA);

[0141] 16 g of methacrylic acid (MAA); and

[0142] 0.94 g of Na₂CO₃ diluted in 100 g of water.

[0143] The system was maintained at this temperature for a further twohours. Next, 1.46 g of t-butyl perbenzoate were added. Then thefollowing were introduced over one hour (until the end of the reaction):0.59 g of erythorbic acid diluted in 47 g of water.

[0144] After cooling to room temperature, the polymer obtained wasanalysed. The results of the analysis were as follows:

[0145] pH=4.6

[0146] M_(n)=13,300 g/mol

[0147] M_(w)/M_(n)=1.75

[0148] 1.3. Hydrolysis of the Diblock Copolymer

[0149] The hydrolysis was carried out in the reactor for synthesizingthe block copolymer emulsion. Introduced into the reactor were:

[0150] 200 g of the above copolymer (section 1.2.), expressed as drymatter (650 g of a 30.8% solution);

[0151] 1900 g of water (in order to adjust the solids content to 10% byweight at the end of hydrolysis).

[0152] Next, the pH was adjusted to a value of 8 using 1N sodiumhydroxide. The temperature was raised to 90° C. and the reaction carriedout under nitrogen.

[0153] With vigorous stirring (160 rpm), 528 g of 2N sodium hydroxide(corresponding to one molar equivalent of sodium hydroxide with respectto ethyl acrylate) were added over 1 hour. After all the sodiumhydroxide had been added, the reaction was maintained under theseconditions for 11 hours.

[0154] The degree of hydrolysis of the acrylate units was measured byproton NMR to be 88 mol %.

[0155] The product recovered at the end of the reaction was atranslucent gel.

Example 2

[0156] Synthesis and Hydrolysis of a poly(styrene/methacrylicacid)-b-poly(ethyl acrylate/methacrylic acid) Diblock Copolymer

[0157] 2.1. Synthesis of a styrene/methacrylic acid Random Copolymer:St/MMA Mass Ratio=95/5

[0158] Introduced into the reactor as a stock, at room temperature, were1112 g of water and 25.36 g of dodecyl sulphate (Texapon K12/96). Themixture obtained was stirred for 30 minutes (175 rpm) under nitrogen.The temperature was then raised to 85° C. and 1.55 g of ammoniumpersulphate (NH₄)₂S₂O₈ diluted in 2.48 g of water were then added.

[0159] At the same time, a mixture comprising:

[0160] 248.04 g of styrene (St),

[0161] 13.99 g of methacrylic acid (MAA), and

[0162] 7.44 g of methyl α-(O-ethylxanthyl)propionate (CH₃CHCO₂Me)SCSOEtstarted to be added.

[0163] The addition was continued for 55 minutes. Fifteen minutes afterthe start of adding the mixture comprising the comonomers and the methylα-(O-ethylxanthyl)propionate, the addition over 45 minutes of 0.56 g ofsodium carbonate Na₂CO₃ dissolved in 100 g of water was started. Afterthe various ingredients had been completely added, the copolymeremulsion obtained was maintained at 85° C. for one hour.

[0164] After cooling to room temperature, 89 g of the polymer emulsionobtained were removed for analysis.

[0165] The results were as follows:

[0166] M_(n)=6500 g/mol

[0167] M_(w)/M_(n)=2.3

[0168] 2.2. Synthesis of the Diblock Copolymer

[0169] The synthesis started with the emulsion copolymer obtained above(section 2.1.). To this were added at 85° C., over one hour:

[0170] 308 g of ethyl acrylate (EtA);

[0171] 16 g of methacrylic acid (MAA); and

[0172] 0.94 g of Na₂CO₃ diluted in 100 g of water.

[0173] The system was maintained at this temperature for a further twohours. Next, 1.46 g of t-butyl perbenzoate were added. Then thefollowing were introduced over one hour (until the end of the reaction):0.59 g of erythorbic acid diluted in 47 g of water.

[0174] After cooling to room temperature, the diblock copolymer emulsionobtained was analysed. The results were as follows:

[0175] pH=5.6

[0176] M_(n)=13,900 g/mol

[0177] M_(w)/M_(n)=1.7

[0178] 2.3. Hydrolysis of the Diblock Copolymer

[0179] The above diblock copolymer (section 2.2.) was hydrolysed.

[0180] The operating method was the same as that in Example 1 (section1.3.) (one molar equivalent of NaOH with respect to the ethyl acrylateunits).

[0181] The degree of hydrolysis obtained was 84 mol %.

[0182] The product recovered at the end of the reaction was atranslucent gel.

Example 3

[0183] Synthesis and Hydrolysis of a poly(styrene/2-hydroxyethylmethacrylate)-b-poly(ethyl acrylate/methacrylic acid) Diblock Copolymer

[0184] 3.1. Synthesis of a styrene/2-hydroxyethyl methacrylate RandomCopolymer: St/HEMA Mass Ratio=95/5

[0185] The experimental protocol was the same as that described inExample 2 (section 2.1.) except that the methacrylic acid was replacedwith an equal amount by weight of 2-hydroxyethyl methacrylate (HEMA). Atthe end of polymerization, an emulsion copolymer was obtained, 89 g ofwhich were removed for analysis.

[0186] The analysis was as follows:

[0187] M_(n)=6400 g/mol

[0188] M_(w)/M_(n)=2.2

[0189] 3.2. Synthesis of the Diblock Copolymer

[0190] The synthesis started with the emulsion copolymer obtained above(section 3.1.). To this were added at 85° C., over one hour:

[0191] 308 g of ethyl acrylate (EtA);

[0192] 16 g of methacrylic acid (MAA); and

[0193] 0.94 g of Na₂CO₃ diluted in 100 g of water.

[0194] The system was maintained at this temperature for a further twohours. Next, 1.46 g of t-butyl perbenzoate were added. Then thefollowing were introduced over one hour (until the end of the reaction):0.59 g of erythorbic acid diluted in 47 g of water.

[0195] After cooling to room temperature, the polymer obtained wasanalysed. The results were as follows:

[0196] pH=5.1

[0197] M_(n)=13,000 g/mol

[0198] M_(w)/M_(n)=1.8

[0199] 3.3. Hydrolysis of the Diblock Copolymer

[0200] The above diblock copolymer (section 3.2.) was hydrolysed.

[0201] The operating method was the same as that in Example 1 (section1.3.) (one molar equivalent of NaOH with respect to the EtA units).

[0202] The degree of hydrolysis obtained was 90 mol %.

Example 4

[0203] Synthesis and Hydrolysis of a poly(styrene/methacrylicacid)-b-poly(ethyl acrylate/methacrylic acid) Diblock Copolymer

[0204] 4.1. Synthesis of a styrene/methacrylic acid Random Copolymer:St/MMA Mass Ratio=90/10.

[0205] Introduced into the reactor as a stock, at room temperature, were1178 g of water and 25.36 g of dodecyl sulphate (Texapon K12/96). Themixture obtained was stirred for 30 minutes (175 rpm) under nitrogen.Next, the temperature was raised to 83° C. and a mixture 1 was added,this comprising:

[0206] 24.8 g of styrene (St);

[0207] 2.72 g of methacrylic acid (MAA); and

[0208] 7.42 g of xanthate (CH₃CHCO₂Me)SCSOEt. The mixture was heated to85° C. and then 1.55 g of ammonium persulphate (NH₄)₂S₂O₈ diluted in2.48 g of water were introduced.

[0209] At the same time, the addition of a mixture 2 comprising:

[0210] 223.24 g of styrene (St) and

[0211] 24.88 g of methacrylic acid (MAA) was started.

[0212] The addition was continued for 55 minutes. Fifteen minutes afterthe comonomer mixture 2 had been added, the addition over 45 minutes of0.56 g of sodium carbonate Na₂CO₃ dissolved in 100 g of water wasstarted. After the various ingredients had been completely added, thecopolymer emulsion obtained was maintained at 85° C. for one hour.

[0213] After cooling to room temperature, 91 g of the emulsion wasremoved for analysis.

[0214] The results of the analysis were as follows:

[0215] M_(n)=6300 g/mol

[0216] M_(w)/M_(n)=2.1

[0217]4.2. Synthesis of the Diblock Copolymer

[0218] The synthesis started with the emulsion copolymer obtained above(section 4.1.). To this were added at 85° C., over one hour:

[0219] 308 g of ethyl acrylate (EtA);

[0220] 16 g of methacrylic acid (MAA); and

[0221] 0.94 g of Na₂CO₃ diluted in 100 g of water.

[0222] The system was maintained at this temperature for a further twohours. Next, 1.46 g of t-butyl perbenzoate were added. Then thefollowing were introduced over one hour (until the end of the reaction):0.59 g of erythorbic acid diluted in 47 g of water.

[0223] After cooling to room temperature, the polymer obtained wasanalysed. The results were as follows:

[0224] M_(n)=13,700 g/mol

[0225] M_(w)/M_(n)=1.8

[0226] 4.3. Hydrolysis of the Diblock Copolymer

[0227] The operating method was the same as that in Example 1 (section4.3.) (one molar equivalent of NaOH with respect to the EtA units).

[0228] The degree of hydrolysis obtained was 90 mol %.

[0229] The product recovered at the end of the reaction was atranslucent gel.

Example 5

[0230] Synthesis and Hydrolysis of a poly(styrene/methacrylicacid/2-hydroxyethyl methacrylate)-b-poly(ethyl acrylate/methacrylicacid) Diblock Copolymer

[0231] This diblock copolymer was synthesized in the same manner as inExample 4.

[0232] The styrene/methacrylic acid/2-hydroxyethyl methacrylate randomcopolymer obtained had the following characteristics:

[0233] mass ratios: St/MAA/HEMA=80/10/10;

[0234] M_(n)=6900 g/mol;

[0235] M_(w)/M_(n)=2.3.

[0236] Starting from this copolymer, a diblock copolymer was synthesizedby polymerizing an ethyl acrylate/methacrylic acid mixture having anEtA/MAA mass ratio of 95/5.

[0237] The diblock copolymer had the following characteristics:

[0238] pH=5.1;

[0239] M_(n)=13,800 g/mol;

[0240] M_(w)/M_(n)=1.7.

[0241] The diblock copolymer was partially hydrolysed to a degreecorresponding to 83 mol %.

Example 6

[0242] Synthesis and Hydrolysis of a poly(styrene/ethylacrylate)-b-poly(ethyl acrylate/methacrylic acid) Diblock Copolymer

[0243] This diblock copolymer was synthesized in the same way as inExample 4.

[0244] The styrene/ethyl acrylate random copolymer obtained had thefollowing characteristics:

[0245] St/EtA mass ratio=80/20;

[0246] M_(n)=7400 g/mol;

[0247] M_(w)/M_(n)=2.2.

[0248] Starting from this copolymer, a diblock copolymer was synthesizedby polymerizing an ethyl acrylate/methacrylic acid mixture having anEtA/MAA mass ratio of 95/5.

[0249] The diblock copolymer had the following characteristics:

[0250] pH=5.1;

[0251] M_(n)=14,200 g/mol;

[0252] M_(w)/M_(n)=1,9

[0253] The diblock copolymer was partially hydrolysed to a degreecorresponding to 90 mol %.

Example 7

[0254] Synthesis and Hydrolysis of a styrene-b-poly(ethylacrylate/methacrylic acid) Diblock Copolymer

[0255] This diblock copolymer was synthesized in the same manner as inExample 4.

[0256] The styrene polymer obtained had the following characteristics:

[0257] M_(n)=2600 g/mol;

[0258] M_(w)/M_(n)=2.4.

[0259] Starting from this polymer, a diblock copolymer was synthesizedby polymerizing an ethyl acrylate/methacrylic acid mixture having anEtA/MAA mass ratio of 95/5.

[0260] The diblock copolymer had the following characteristics:

[0261] pH 5.1;

[0262] M_(n)=17,700 g/mol;

[0263] M_(w)/M_(n)=2.7.

[0264] The diblock copolymer was partially hydrolysed to a degreecorresponding to 87 mol %.

B—Properties of the Block Copolymers of Examples 1 to 7 Example 8

[0265] Diblock Copolymers Comprising a Predominantly Hydrophobic Blockand a Water-Soluble Block

[0266] The hydrolysed block copolymers of Examples 1 to 6 had:

[0267] a water-soluble block and

[0268] a predominantly hydrophobic block.

[0269] After hydrolysis, these polymers were washed by dialysis againstwater. Depending on the analytical test to which they were subjected,they were then:

[0270] either concentrated by freeze drying and then redispersed;

[0271] or diluted in millipore water so as to bring them to the desiredconcentration.

[0272] The pH was adjusted to 9.

[0273] Test for the Presence of a Viscoelastic Gel

[0274] All these block copolymers form a translucent gel at lowconcentration in water. The critical weight concentration at which theyform a gel in solution, called the “critical gelling concentration” isgiven in Table 1. This concentration is that for which the elasticmodulus G′ becomes greater than the viscous modulus (G″). Themeasurements are given in Table 1. TABLE 1 Critical gelling Exampleconcentration 1 4% by weight 2 4% by weight 3 5% by weight 4 2% byweight 5 3% by weight 6 4% by weight

[0275] In the case of Examples 2, 4 and 5, the values of the elasticmodulus (G′) and the viscous modulus (G″) were measured using aRhéométrixe ARES rheometer under the following conditions:

[0276] frequencies between 10⁻² and 10² rad/s;

[0277] 20% deformation,

[0278] 5% concentration by weight (solid content) of polymer.

[0279] The measurements are given in Table 2. TABLE 2 Example G′ (Pa) G″(Pa) 2 60 10 4 400 20 5 100 10

[0280] It may be seen that the elastic modulus is always greater thanthe viscous modulus. The strongest gel is that of Example 4 (highestelastic modulus) which also has the lowest critical gellingconcentration.

[0281] Test for the Presence of Micelles

[0282] The block copolymers of Examples 1 to 5 were dissolved in waterto a concentration of 10⁻²%.

[0283] The size of the polystyrene-based hydrophobic core of themicelles was determined by low-angle neutron scattering, after dilutingthe polymers in heavy water (D₂O) to 10⁻²% and by applying the knownconventional processing to the scattering spectra.

[0284] Thus, it was found that, for the examples below, the polystyrenehydrophobic core of the micelles was essentially spherical. In Table 3,the radii deduced from the so-called “Guinier” and “Porod” processingare indicated.

[0285] The aggregation number, which corresponds to the number ofdiblocks participating in one micelle, is calculated from the volume ofthe hydrophobic core of the micelles. It is given in Table 3 below,calculated from the value of the “Guinier” radius. TABLE 3 AggregationExample “Guinier” radius “Porod” radius number 2  10 nm 14 nm 190 4   8nm 11 nm 100 5 6.5 nm  9 nm 50

[0286] The value of the hydrophobic core radius and the spherical shapewere confirmed in the case of Examples 2 and 4 by cryo-microscopy(transmission electron microscopy carried out on a frozen sample). Smallspherical particles from 15 to 20 nm in diameter attributed to thepolystyrene core were observed.

[0287] Test for the Presence of Associated Micelles

[0288] These solutions 1, 2, 4 and 5 were analysed by quasielastic lightscattering using a Brookhaven scattering set-up (BI-200SM goniometer andBI-900AT correlator) at an angle of 90° and by applying the processingusing the so-called “Contin” apparatus. By measuring the autocorrelationspectrum, a “slow” scattering coefficient associated with the existenceof large objects a few hundred nm in size was deduced. The size of theobjects for a concentration of 10⁻²% is given in Table 4. TABLE 4Example Size of the objects 1 185 nm 2 175 nm 4 320 nm 5 100 nm

[0289] Since the maximum theoretical size of a 15,000 mass diblock insolution is less than 100 nm, these large objects therefore result fromthe association of the diblocks among themselves in the form of micellesor from the association of micelles among themselves, probably byassociation of the hydrophobic units of the water-soluble blocks.

Example 9

[0290] Diblock Polymers Comprising a Completely Hydrophobic Block and aWater-Soluble Block

[0291] Copolymer according to Example 7:

[0292] This block copolymer dissolved in water formed a translucent gelat low concentration: the value of the critical gelling concentrationwas 3% by weight.

[0293] This copolymer was analysed using the following techniques:

[0294] by neutron scattering, it was found that the sphericalhydrophobic polystyrene core had a “Guinier” radius of 8.6 nm,

[0295] the shape and the size were confirmed by an electroncryo-micrograph,

[0296] the size of the object determined by quasielastic lightscattering was 337 nm and its fractal dimension determined by staticlight scattering was 1.

[0297] We therefore obtained in the case of a very asymmetric diblock(17/83) with a completely hydrophobic block and a partially hydrophilicblock the same type of property as with a partiallyhydrophilic/partially hydrophobic symmetric block (50/50) (Examples 1 to6).

[0298] With a completely hydrophobic block, it is necessary to have avery asymmetric diblock (small hydrophobic block) in order to maintainthe solubility and the gelling.

C. Block Copolymer Synthesis (Examples 10 and 11) Example 10

[0299] Synthesis and Hydrolysis of a poly(styrene/methacrylicacid)-b-poly(ethyl acrylate/methacrylic acid) Diblock Copolymer

[0300] 10.1. Synthesis of a styrene/methacrylic acid Random Copolymer.

[0301] St/MAA mass ratio: 98/2; theoretical mass: M_(n)=2000 g/mol.

[0302] Introduced into a reactor as a stock, at room temperature, were682.5 g of water, 8.54 g of sodium dodecyl sulphate and 0.189 g ofsodium carbonate Na₂CO₃. The mixture obtained was stirred for 30 minutes(190 rpm) under nitrogen. Next, the temperature was raised to 75° C.before adding a mixture 1 comprising:

[0303] 5.19 g of styrene (St);

[0304] 0.105 g of methacrylic acid (MAA); and

[0305] 5.51 g of xanthate (CH₃CHCO₂Me)SCSOEt.

[0306] The mixture was heated to 85° C. and then 1.21 g of ammoniumpersulphate (NH₄)₂S₂O₈ were introduced.

[0307] At the same time, the addition of a mixture 2 comprising:

[0308] 46.78 g of styrene (St) and

[0309] 0.945 g of methacrylic acid (MAA) was started.

[0310] The addition was continued for 60 minutes. After completeaddition of the various ingredients, the copolymer emulsion obtained wasmaintained at 85° C. for one hour.

[0311] 10.2. Synthesis of the Diblock Copolymer. EtA/MAA Mass Ratio:98/2; Theoretical Mass M_(n)=21,468 g/mol.

[0312] The synthesis started with the emulsion copolymer obtained above(section 10.1.), into which were introduced 0.576 g of ammoniumpersulphate (NH₄)₂S₂O₈ diluted in 10 g of water.

[0313] To this were added at 85° C., over one hour:

[0314] 481.9 g of ethyl acrylate (EtA);

[0315] 9.8 g of methacrylic acid (MAA); and

[0316] 0.545 g of Na₂CO₃ diluted in 150 g of water.

[0317] The system was maintained at this temperature for a further threehours.

[0318] 10.3. Hydrolysis of the Diblock Copolymer

[0319] Specimen 10.3.a : The above copolymer was hydrolysed. Introducedinto the reactor were:

[0320] 30 g of the above copolymer (section 10.2.) expressed as drymatter (40.2% of 74.6 g);

[0321] 157.4 g of water (in order to adjust the solids content to 10% byweight at the end of hydrolysis).

[0322] The temperature was raised to 90° C. While stirring vigorously,67.9 ml of 2N sodium hydroxide (corresponding to 0.51 molar equivalentof sodium hydroxide with respect to ethyl acrylate) were added over 1hour. After the sodium hydroxide had been completely added, the reactionwas maintained under these conditions for 24 hours. The productrecovered at the end of the reaction was a translucent gel. Thehydrolysis rate determined by NMR is about 44%,

[0323] Specimen 10.3.b : The operating method was the same as that forSpecimen 10.2. The amount of sodium hydroxide added corresponded to 0.66molar equivalent of sodium hydroxide with respect to ethyl acrylate. Theproduct recovered at the end of the reaction was a translucent gel. Thehydrolysis rate, determined by NMR is about 61%.

[0324] Specimen 10.3.c : The operating method was the same as that forSpecimen 10.2. The amount of sodium hydroxide added corresponded to 0.76molar equivalent of sodium hydroxide with respect to ethyl acrylate. Theproduct recovered at the end of the reaction was a translucent gel. Thehydrolysis rate, determined by NMR is about 72%.

[0325] Specimen 10.3.d : The operating method was the same as that forSpecimen 10.2. The amount of sodium hydroxide added corresponded to 0.9molar equivalent of sodium hydroxide with respect to ethyl acrylate. Theproduct recovered at the end of the reaction was a translucent gel. Thehydrolysis rate, determined by NMR is about 79%.

[0326] Specimen 10.3.e : The operating method was the same as that forSpecimen 10.2. The amount of sodium hydroxide added corresponded to 2molar equivalents of sodium hydroxide with respect to ethyl acrylate.The product recovered at the end of the reaction was a translucent gel.The hydrolysis rate, determined by NMR is higher than 95% and lesserthan 98%

Example 11

[0327] Synthesis and Hydrolysis of a poly(styrene/methacrylicacid)-b-poly(ethyl acrylate/methacrylic acid)-b-poly(styrene/methacrylicacid) Triblock Copolymer of 2000-19468-500 Theoretical Mass

[0328] 11.1. Synthesis of a styrene/methacrylic acid Random Copolymer.St/MAA Mass Ratio: 98/2.

[0329] The experimental protocol was identical to that described inExample 10, section 10.1.

[0330]11.2. Synthesis of the Diblock Copolymer. EtA/MAA Mass Ratio:98/2; Theoretical Mass M_(n): 21,468 g/mol.

[0331] The experimental protocol was identical to that described inExample 10, section 10.2.

[0332]11.3. Synthesis of the Triblock Copolymer. PS/MAA Mass Ratio inthe 3rd block: 98/2; Theoretical Mass M_(n): 21,968 g/mol.

[0333] Starting with 968 g of the diblock copolymer obtained above(section 11.2), 0.032 g of sodium carbonate Na₂CO₃ diluted in 5 g ofwater and 0.2878 g of ammonium persulphate (NH₄)₂S₂O₈ diluted in 10 g ofwater were introduced.

[0334] Over one hour, the following were added at 85° C.:

[0335] 9 g of styrene (St);

[0336] 0.173 g of methacrylic acid (MAA).

[0337] The system was maintained at this temperature for a further onehour.

[0338] 11.4. Hydrolysis of the Triblock Copolymer

[0339] The above copolymer was hydrolysed according to the protocoldescribed for Specimen 10.3.a. The amount of sodium hydroxide addedcorresponded to 2 molar equivalents of sodium hydroxide with respect toethyl acrylate. The product recovered at the end of the reaction was atranslucent gel. The hydrolysis rate is higher than 95% and lesser than98%.

D—Properties of the Block Copolymers (of Examples 10 and 11) Example 12

[0340] Diblock Copolymers Comprising a Predominantly Hydrophobic Blockand a Water-Soluble Block. Variation in the Number of Hydrophobic Unitsin the Water-Soluble Block.

[0341] After hydrolysis, the copolymers 10.3.a, 10.3.b, 10.3.c and10.3.d were diluted in millipore water in order to bring them to thedesired concentration. The copolymer 10.3.e was washed by dialysisagainst water and then diluted in millipore water in order to bring itto the desired concentration.

[0342] In the case of the copolymers 10.3.a, 10.3.b, 10.3.c, 10.3.d and10.3.e, the values of the elastic modulus (G′) and the viscous modulus(G″) were measured using a Rhéométrix SR200 rheometer under thefollowing conditions:

[0343] frequencies between 10⁻² and 10² rad/s,

[0344] 5 or 10% deformation,

[0345] 2% concentration by weight (solid content) of polymer.

[0346] The values obtained at a frequency of 1 rad/s are given in Table5. TABLE 5 Example G′ (Pa) G″ (Pa) 10.3.a 11.9 4.4 10.3.b 16.8 5.010.3.c 9.1 4.1 10.3.d 1.8 1.2 10.3.e 0.65 0.40

[0347] From Table 5, it appears that the elastic modulus exhibits amaxima for a hydrolysis rate of about 60%.

Example 13

[0348] Triblock Copolymer Comprising a Predominantly Hydrophobic Block,a Water-Soluble Block and a Predominantly Hydrophobic Block.

[0349] After hydrolysis, the copolymer of Example 11.4 was washed bydialysis against water and then diluted in millipore water in order tobring it to the desired concentration.

[0350] The values of the elastic modulus and the viscous modulus weredetermined using the same operating method as in Example 12.

[0351] The values obtained at a frequency of 1 rad/s are given in Table6. TABLE 6 Example G′ (Pa) G″ (Pa) 11.4 30.2 3.6 10.3.e 0.65 0.40

[0352] From table 6, it appears that using a triblock provides asubstantial increase of the elastic modulus.

1. A block copolymer comprising at least one block water-soluble innature and containing hydrophobic units and at least one blockpredominantly hydrophobic in nature, and, in solution in water, forminga viscoelastic gel.
 2. A block copolymer according to claim 1, whereinthe predominantly hydrophobic block has hydrophilic units in an amountof between 0 and less than 33% by weight, with respect to the totalweight of the units of the hydrophobic block.
 3. A block copolymeraccording to claim 1, wherein the predominantly hydrophobic block hashydrophilic units in an amount of between 1 and less than 33% by weight,with respect to the total weight of the units of the hydrophobic block.4. A block copolymer according to claim 2, wherein the predominantlyhydrophobic block has hydrophilic units in an amount of between 2 and15%, with respect to the total weight of the units of the hydrophobicblock.
 5. A block copolymer according to claim 1, wherein the blockwater-soluble in nature has hydrophobic units in an amount of less than70% by weight, with respect to the total weight of the units of thewater-soluble block.
 6. A block copolymer according to claim 1, whereinthe block water-soluble in nature has hydrophobic units in an amount ofbetween 50 and 25%, with respect to the total weight of the units of thewater-soluble block.
 7. A block copolymer according to claim 5, whereinthe block water-soluble in nature has hydrophobic units in an amount ofat least 1% by weight.
 8. A block copolymer according to claim 1,wherein the block predominantly hydrophobic in nature is a completelyhydrophobic block.
 9. A block copolymer according to claim 1, whereinthe mass ratio of the blocks predominantly hydrophilic in nature to theblocks predominantly hydrophobic in nature is between 95/5 and 20/80,preferably between 90/10 and 40/60.
 10. A block copolymer according toclaim 1, wherein the copolymer is at a concentration of between 0.1% and10% by weight.
 11. A block copolymer according to claim 1, wherein atleast one of the said blocks is a copolymer coming from thecopolymerization of hydrophilic and hydrophobic monomers.
 12. A blockcopolymer according to claim 11, wherein the amounts of hydrophilic andhydrophobic units in each of the said blocks are controlled by therespective contents of hydrophilic monomers and of hydrophobic monomersupon polymerization of the blocks.
 13. A block copolymer according toclaim 1, wherein at least one of the said blocks is a copolymerprepared: from the polymerization of monomers that may be renderedhydrophilic by hydrolysis, and optionally of non-hydrolysablehydrophobic monomers and of hydrophilic monomers, and then from thehydrolysis of the polymer obtained.
 14. A block copolymer according toclaim 13, wherein the amounts of hydrophilic and hydrophobic units ineach of the said blocks are controlled by the amount of monomers thatcan be rendered hydrophilic by hydrolysis and by the degree ofhydrolysis.
 15. A block copolymer according to claim 13, wherein thehydrophobic monomers are: vinylaromatic monomers; diolefins; or alkylacrylates and methacrylates, the alkyl group of which contains from 1 to10 carbon atoms.
 16. A block copolymer according to claim 13, whereinthe hydrophilic monomers are: ethylenically unsaturated carboxylicacids; acrylamide,(N-methylacrylamide, N-isopropylacrylamide),methacrylamide, polyethylene glycol methacrylate, polyethylene glycolacrylate; sodium 2-acrylamido-2-methylpropanesulphonate (SAMPS), sodiumstyrenesulphonate or sodium vinylsulphonate.
 17. A block copolymeraccording to claim 13, wherein the monomers that can be renderedhydrophilic by hydrolysis are: methyl acrylate, ethyl acrylate,hydroxyethyl acrylate, hydroxyethyl methacrylate, tert-butyl acrylate;vinyl acetate; quaternized 2-dimethylaminoethyl methacrylate andacrylate; acrylamide or methacrylamide.
 18. A block copolymer accordingto claim 1, being a diblock copolymer or a triblock copolymer having ablock water-soluble in nature flanked by two blocks predominantlyhydrophobic in nature.
 19. A block copolymer according to claim 1,wherein it is a diblock copolymer comprising a block water-soluble innature and a block predominantly hydrophobic in nature, the blockwater-soluble in nature containing acrylic acid (AA) units and ethylacrylate (EtA) units and the block predominantly hydrophobic in naturecontaining styrene (St) units and methacrylic acid (MAA) or hydroxyethylmethacrylate (HEMA) units.
 20. A block copolymer according to claim 19,wherein the block water-soluble in nature comes: from the polymerizationof methacrylic acid (MAA) and of ethyl acrylate (EtA) in an EtA/MAAweight ratio from 90/10 to 99/1, and then from the hydrolysis of thepolymer obtained to a degree of at least 50 mol % up to 95 mol %.
 21. Ablock copolymer according to claim 20, wherein the block predominantlyhydrophobic in nature comes from the polymerization of a monomer mixturecontaining at least 80% by weight styrene.
 22. A block copolymeraccording to claim 1, having a molecular mass of at most 100,000 g/mol.23. A process for preparing a block copolymer as defined in claim 1,prepared from a so-called living or controlled polymerization process.24. A process for the preparation of a block copolymer as defined inclaim 1, comprising the steps of: a) bringing into contact with oneanother: at least one ethylenically unsaturated monomer, at least onesource of free radicals and at least one compound of formula (I):

 wherein: R represents an R²O—, R²R′²N— or R³— group, where: R² and R′²,which are identical or different, represent (i) an alkyl, acyl, aryl,alkene or alkyne group or (ii) a saturated or unsaturated, possiblyaromatic, carbocycle or (iii) a saturated or unsaturated heterocycle,these groups and rings (i), (ii) and (iii) possibly being substituted,R³ represents H, Cl, an alkyl, aryl, alkene or alkyne group, a saturatedor unsaturated ring, a saturated or unsaturated heterocycle, analkylthio, alkoxycarbonyl, aryloxycarbonyl, carboxy, acyloxy, carbamoyl,cyano, dialkylphosphonato, diarylphosphonato, dialkylphosphinato ordiarylphosphinato group, or a polymer chain, R¹ represents (i) anoptionally substituted alkyl, acyl, aryl, alkene or alkyne group or (ii)an optionally substituted or aromatic, saturated or unsaturated,carbocycle or (iii) an optionally substituted, saturated or unsaturated,heterocycle, or a polymer chain; to obtain a polymer b) repeating abovestep a) at least once, using: monomers differing from those used in stepa), and in lieu of the precursor compound of formula (I), the polymerobtained in the previous step b); c) optionally, hydrolysing thecopolymer obtained in step b): and d)recovering said copolymer.
 25. Aprocess according to claim 24, wherein the compound of formula (I) is adithiocarbonate chosen from compounds of the following formulae (IA),(IB) and (IC):

wherein: R² and R² represent (i) an alkyl, acyl, aryl, alkene or alkynegroup, or (ii) an optionally aromatic, saturated or unsaturated,carbocycle or (iii) a saturated or unsaturated heterocycle, these groupsand rings (i), (ii) and (iii) possibly being substituted; R¹ and R^(1′)represent (i) an optionally substituted alkyl, acyl, aryl, alkene oralkyne group or (ii) an optionally substituted or aromatic, saturated orunsaturated, carbocycle or (iii) an optionally substituted, saturated orunsaturated, heterocycle, or a polymer chain; p is between 2 and
 10. 26.A process for controlling the hydrophilic/hydrophobic balance ofamphiphilic block copolymers having at least one block coming from thepolymerization of hydrophilic monomers and at least one block comingfrom the polymerization of hydrophobic monomers, wherein: hydrophilicunits are introduced into the block coming from the polymerization ofhydrophobic monomers, and/or hydrophobic units are introduced into theblock coming from the polymerization of hydrophilic monomers.
 27. agelling agent or a thickener comprising a copolymer as defined in claim1.